This invention relates to hydraulic systems in which pressurized fluid is used for operating hydraulic jacks or other fluid-driven mechanisms and more particularly to the hydraulic tanks which contain the operating fluid supply for such systems.
Many hydraulic systems have components which require different amounts of operating fluid at different stages of operation. On earthmoving vehicles for example, various load-manipulating elements may be operated by fluid cylinders or jacks of the form having a piston which is extended within a cylinder by oil pressure. As the jack extends, the amount of oil in the jack increases and oil is released as the jack contracts. Although in most cases not all the components of a given system are extended to their maximum oil-containing volume at the same time, this can occur on occasion and thus the volume of oil available to the system should be at least equal to the maximum oil-containing capacity of all such displacement-type components. In practice it may be desirable that somewhat more oil be available to compensate for possible leakage and to compensate for the effects of temperature changes in varying the volume of a given amount of oil.
To meet these requirements, hydraulic systems are customarily provided with a storage means or hydraulic tank from which oil is withdrawn as needed when the displacement of components of the system increase and to which oil is returned as the displacement of such components is decreased. In most instances oil is withdrawn from the hydraulic tank by a pump, which pressurizes the fluid, and is later returned to the tank through drain lines.
One form of hydraulic tank simply has an oil outlet at or near the lower portion of the tank while the upper portion of the tank is vented to the atmosphere. Such tanks are subject to several serious problems and impose design restrictions on the associated hydraulic system which may not necessarily be desirable. The intermixing of air and oil which can occur in such a tank detracts from the working efficiency of the associated system, may cause rapid deterioration of the oil and imposes requirements for baffles and diffusers and the like in oil return lines. In most cases such a tank must necessarily be situated above the pump to which it is connected. In instances where the tank may not always be maintained in a precisely horizontal position, such as on an earthworking vehicle, restrictions on the shape of the tank are present as it must be made sufficiently high in relation to its width to assure that the oil in the tank remains adjacent the outlet when the tank is tilted. Such a tank must be large and costly in order to retain a volume of oil exceeding the maximum requirements of the associated hydraulic system and in order to deliver and receive oil at sufficiently rapid rates to accommodate to the needs of the system. Cooling problems are aggravated in that oil is not usually in contact with all of the inner surface of the tank. Further, such a tank is not ideally adapted to the use of a screen or filter between the tank and the pump which draws fluid from the tank. Sediment tends to collect on the bottom of the tank and since the outlet is necessarily at the bottom, a screen or filter in the oil withdrawal line tends to clog very rapidly. The pressure tending to force oil through such a filter is relatively low.
Some of the foregoing problems, but by no means all, may be alleviated if the tank is sealed as is a common practice at this time. Using conventional constructions, this requires a relatively large-sized tank to provide a sufficient air volume to avoid excessive pressure changes when substantial amounts of oil are withdrawn or returned to the tank. The conventional sealed tank is still subject to the configuration and location restrictions, aeration complications and many of the other problems discussed above.
To alleviate still more of these problems hydraulic tanks have heretofore been constructed in which the air volume within the tank is completely sealed from the oil volume by flexible fluid-tight means which can expand and contract as oil is withdrawn and returned. Copending application Ser. No. 443,303 of Ralph W. Matthews filed Feb. 19, 1974 now U.S. Pat. No. 3,935,882 and entitled "Hydraulic Tank Reservoir Pressure and Vacuum Stabilizer System" discloses such a system. Problems arising from the intermixing of air and oil are greatly reduced but may not be wholly eliminated. Aeration can still occur from leakage past seals and joints in the tank or associated hydraulic system particularly when the internal fluid pressure drops slightly below atmospheric due to small negative pressures arising from fluid flows.
In part to alleviate the last-mentioned problem and also to free the tank configuration from restrictions on the location of the oil outlet, some prior tank constructions have employed an expandable and contractable internal bladder to contain the air volume with the bladder being charged with compressed air. While these designs alleviate still more of the problems discussed above, prior tanks of this kind are subject to a further problem in that the tank pressure varies substantially. As oil or the like is withdrawn, the bladder expands and therefore the pressure within the tank including the oil pressure decreases. As oil is returned the bladder contracts and tank pressure necessarily rises. This variable base pressure can have adverse effects on the operation of the pump and on the hydraulic jacks and other components of the hydraulic system. While pressure fluctuation can be counteracted to a limited extent by increasing the size of the tank, this in itself can be a significant disadvantage as discussed above.